The present invention relates to optical fiber transmission technology by spectrum division multiplexing, and more particularly it relates to a system enabling wavelength band separation to be improved in a transmission mode where a large number of channels having the same destination are transported by a respective sequence of different wavelengths that are regularly distributed in the spectrum, all of these wavelengths being transported by the same fiber and constituting a so-called “band”. In other words, the spectrum of a band is in the form of a comb of finite width, and with teeth that are regularly spaced apart. A plurality of bands may be interlaced if the spectrum space between two successive channels in a band is at least equal to the space needed for a channel.
In recent years, enormous demand for bandwidth has been initiated by the deployment of modern forms of telecommunications, in particular the Internet and its main application the world wide web, but also all of the private networks of businesses and various other organizations, not forgetting wireless communications, in particular mobile telephony which, in practice, requires high performance terrestrial infrastructure to meet customer expectations. To satisfy this demand, those in charge of implementing the networks needed for deploying such novel forms of communication have rapidly needed to have recourse to transporting signals carrying information in optical form in order to benefit firstly from the low cost of fibers themselves and secondly from the very high data rates that can be achieved in spite of transmission distances that can be measured in kilometers or even tens or hundreds of kilometers, without any need for signal regeneration. Attenuation is very low, in particular in so-called “single mode”, fibers, when compared with the level of attenuation that can be obtained for electrical transmission using copper, for example. Furthermore, optical transmission avoids all problems associated with electromagnetic disturbances which require expensive protection circuits and can lead to frequent errors in transmission.
A first step in the use of optical fibers consisted essentially in point-to-point links between two nodes of a network. During that first step, transport took place over a single wavelength, usually around 1550 nanometers (nm) since that is the wavelength that is most suitable for long distance transport. Although optical signals can be modulated up to very high frequencies, which are expressed in gigabits per second (Gb/s, i.e. 109 bits per second), the quantity of information that needs to be transported rapidly outgrew the available capacity. Although the fiber itself is of low cost, deploying it can require a large amount of labor and can be extremely expensive. Instead of deploying larger numbers of optical fibers when the capacity of an installed network becomes insufficient, a solution has been found in making better use of those fibers that are already in place. The technique of wavelength division multiplexing (WDM) makes it possible to increase the number of completely independent transmission channels within a single physical fiber by transmitting different wavelengths along the same fiber. In other words, by transmitting light rays of different “colors”, the bandwidth of a single fiber is increased correspondingly. The technique of dense WDM (DWDM) quickly supplanted WDM and now makes it possible to multiplex hundreds of channels, or even more.
It has been shown that, under most circumstances, routing wavelengths in groups, i.e. bands (as contrasted with conventional routing by individual wavelengths) leads to a reduction in the overall cost of an optical network insofar as it enables the cost of the filter and switch devices present in nodes to be shared between the various different wavelengths making up a band. Furthermore, amongst the band configurations that are suitable for use for routing purposes, contiguous or adjacent wavelength bands present a major advantage over interlaced wavelength bands in that they use simpler filter devices that are less expensive and also more tolerant from the point of view of the physical constraints they impose (in contrast to interlaced bands which require filter devices that filter each wavelength individually).
Such fibers are used for interconnecting optical switching equipment situated at the various nodes of a network. An essential device at a network node is then an optical add/drop multiplexer (OADM). As its name suggest, it serves to remove or demultiplex and to insert or multiplex local traffic optically (e.g. traffic that is local to an inlet/outlet point for a secondary network), while allowing the remainder of the traffic to continue its journey towards other nodes of the network. This assumes that at least one wavelength carrying local information (one channel) can be dropped and added. In practice, for the reasons mentioned above, it is usually wavelength bands (sequences of channels) that need to be dropped and added in this way in order to be able to exchange a sufficient quantity of information with local applications.
This is shown in FIG. 1, which shows an ordinary optical demultiplexer 100 separately extracting three wavelength bands of a multiplex constituted by a sequence of channels distributed at regular intervals within the spectrum:                a band 110 constituted by four channels carried respectively by a first group of four wavelengths;        a band 120 constituted by four channels carried respectively by a second group of four wavelengths; and        a band 130 constituted by four channels carried respectively by a third group of four wavelengths.        
The bands 110 and 120 are separated by a band 139 corresponding to two channels, but which is not used for transmitting signals. The bands 120 and 130 are separated by a band 140 corresponding to two channels, but which is not used for transmitting signals. If ordinary band demultiplexing equipment is used, the bands 110, 120, 130 cannot be extracted without it being necessary to sacrifice so-called “inter-band” channels between the bands 110 and 120 and between the bands 120 and 130. This is due to the great proximity of contiguous wavelengths that can be transported in so-called DWDM mode (involving spectrum separations between channels of 100 GHz, 50 GHz, or even 25 GHz), and to the limitations inherent to the spectral discrimination ability (also known as selectivity) of ordinary band demultiplexing equipment, such as the demultiplexer 100.
In FIG. 1, the passband of the ordinary demultiplexer 100 is represented by a trapezoid 150 having sides of slope that is not sufficiently steep. This passband is broad enough to cover the channels of a wavelength band (four channels in this example) but is of selectivity that is insufficient for effectively eliminating the inter-band channels constituting the bands 139 and 140. These inter-band channels therefore cannot be used, thereby significantly reducing the transport capacity on the optical fiber.